The present invention relates to a magnetic signal detecting apparatus employing a ferromagnetic magnetoresistive element (hereinafter referred to as an MR element) and more particularly to a position detecting apparatus adapted to detect a signal magnetic field indicative of a desired position.
The position detecting apparatus employing an MR element is high in detection sensitivity, so that the utilization of its digital magnetic signal as a detection signal is looked for.
FIG. 1 shows a magnetic field characteristic of the MR element. Its resistance value R varies with a variation of a magnetic field H perpendicular to a detection current i (in the transverse or short direction of the MR element), that is, the resistance value R increases gradually with decrease in the magnetic field H and it increases rapidly as the magnetic field H becomes lower than a certain value. This resistance variation (the curve l.sub.1) differs between cases where the magnetic field H reverses its direction from positive to negative and where the magnetic field H reverses its direction from negative to positive and there are instances where the distortions B are caused by the hysteresis of the magnetic response due to the anisotropic dispersion or the Barkhausen noise. Also, where such distortion is large, the resistance value fails to increase or it decreases as shown by the broken line l.sub.2 in FIG. 1. Also, from the aspect of manufacture there are problems that the postfabrication characteristics of the elements tend to become nonuniform, that the yield is subjected to fluctuation and so on.
With a view to overcoming the foregoing deficiencies, various methods have been investigated in which a bias field is applied in the longitudinal direction of the MR element so as to prevent any instability of a detection output, etc., (e.g., Japanese Patent Unexamined Publication Nos. 16580/83 and 18458/84).
In this type of apparatus, a signal field generating multipole magnet and the MR element are arranged in such a manner that the axial direction of the former and the patterned surface of the latter are parallel to each other. In the case of this conventional apparatus, as shown by the characteristic b in FIG. 3, the peak value position is shifted by the hysteresis characteristic from each N or S pole position in the direction of the rotation and also a distortion in the resistance variation is caused by noise thus reducing the rate of resistance variation .vertline..DELTA.R/R.sub.0 .vertline. to about 2%. Here, R.sub.0 the resistance value when the magnetic field H is zero.